Your client
has nearly all the information she needs, but wants to know how Ice
and Independence Lakes fit in with other typical lakes. Are they considered
average, cleaner, or poorer quality than other lakes? Use Figure 3 to
estimate the "trophic status" of each lake. What are they?

Background
Information: A variety of relatively simple empirical models (i.e. based
on relationships between measured data rather than derived from an understanding
of all the processes) have been developed since the mid-1960's to predict
eutrophication on the basis of phosphorus loadings. The P-loading concept
assumes that algal growth is limited by the availability of phosphorus
in the water and that increased P, which is derived from sewage discharges
and from runoff into lakes and streams, has caused water quality degradation
- but the sources are controllable. Typically, these models are used
to relate the loading rates for P into the lake to summer concentrations
of phosphorus in the lakewater. Then, other empirical relationships
are used that link P to various measures of water quality, such as clarity
(Secchi depth), algae
(chlorophyll) and oxygen depletion in bottom waters.

The Vollenweider
plot shown in Figure 3 was developed by Richard Vollenweider, a Canadian
limnologist. He noted that deeper lakes were generally less susceptible
to phosphorus pollution than shallower lakes. He compiled loading rates,
mean depths and
trophic states
for a set of hundreds of temperate lakes
around the world and then visually drew the lines separating the lakes
into categories (oligo-, meso- and eutrophic). These plots could then
be used by planners to predict how new developments (which cause varying
amounts of "new" P-loading) would impact a lake, or by lake managers
to determine how to best reduce P-loading to improve an already degraded
lake.

Lake Tahoe
and Lake Superior are also shown as examples of extremely deep lakes
with low loading rates.

Figure
3 shows a plot of annual phosphorus loading versus mean depth for selected
U.S. lakes (data from Table 12). The dashed lines were based on a large
data set of the world's lakes and separate them into Eutrophic (productive)
and Oligotrophic (unproductive) lakes on the basis of their depth and
phosphorus imputs from the surrounding watershed.
These loading
rates are simply the total weight of phosphorus entering the lake
each year - mostly from streams, of course.